The invention relates to a bipolar multiple electrolytic cell comprising a diaphragm for the decomposition of alkali-halogenide solutions into lye, halogenide and hydrogen. In particular, the improved construction relates to electrical alkali-chlorine diaphragm cells for obtaining caustic solution, chlorine and hydrogen.
Electrolytic cells of this kind are required to be compact, to be of simple construction, to be capable of carrying a heavy electrical load and thus to be very economical in use. Since the media passing through the electrolytic cell are particularly aggressive, chemically and/or physically, the material of construction must be resistant, and a form of construction is needed that meets the requirements.
Various constructions of multiple electrolytic cells are known in which the electrodes are bipolar and use is made of a diaphragm. In these types of cell the bipolar electrode consists of a metal structure with a diaphragm applied thereto. Thus, for example, the bipolar electrode shown in FIG. 5 of German Pat. No. 1,421,041, as laid open, consists of a sheet of titanium which is coated on the anode side with a noble metal such as platinum, while its other side is uncoated and comprises small projections through which it is connected electrically to a wire mesh, the chamber so formed constituting the cathode. The outer surface of the wire mesh is covered with an asbestos fibre cloth which constitutes the diaphragm. Even distribution of the current in the electrolyte and uniform flow of current in the electrode cannot be achieved with a bipolar electrode of this kind. The uniformly distributed current flowing from the electrode side of the anode, coated with noble metal, passes through the anolyte and the asbestos diaphragm into the wire mesh and from here travels by way of the individual projections on the titanium electrode into the titanium sheet itself and then to the coating of noble metal. The point of contact at the small projections poses a problem. Furthermore, because of the many point contacts at the places where the wires cross, a wire mesh in addition offers resistance to the flow of electric current. The corrosive media in the cell furthermore lead to breaks in contact in the wire mesh through oxide formation. The titanium electrode, unprotected on the cathode side, is exposed to hydrogen and becomes brittle after a short time.
According to German Pat. Nos. 1,592,020 and 2,030,610, as laid open, a bipolar electrode is constituted by a carrier plate, which also acts as a partition for the individual cells, by a plurality of anode plates on one side of the carrier plate, and by a plurality of cathode fingers on the other side of the carrier plate. The anodes are made of a suitable chlorine-resisting material, and the cathodes, which are in the form of metal wire or lattice work structures, are made of iron, nickel or chromium, or of a metal which resists the hydrogen and alkaline compounds. The metal wire mesh is fixed on the carrier plate by means of a supporting device and is covered with a cloth of asbestos fibre which acts as a diaphragm. In order to be able to accommodate the greatest possible area of electrode in a given space, the anode and cathode parts of the electrodes are of comb-like structure and in the assembled condition they extend into each other without touching. Bipolar electrodes of this construction are complicated, comprise a large number of contact points endangered by corrosion, and exhibit uneven current distribution and long current paths in the conductor material, in the anodes and the cathodes, since the electric current passes through some of these in the longitudinal and not the transverse direction. The repair and maintenance of bipolar electrodes of the known construction are not without their problems on account of the many individual parts in each electrode and the inability to check the contact points during use.
Electrodes covered by a coating of an alloy, e.g. steel or titanium, have been used in electrolytic cells. The coating is performed by electroplating. The coating material is a non-ferrous metal, mostly an alloy (see table II of U.S. Pat. No. 3,291,714). The coating alloys according to prior art consist of two or more metals and these inventions are based on the idea that an electrode made from metal, iron or titanium, must be covered by a coating of pure metal or an alloy, which consists of pure metals only and does not contain any metalloids, in order to guarantee an optimum current flow through the electrode. In the above U.S. patent, alloys of molybdenum, nickel, cobalt and tungsten are preferably used for the coating by electroplating.
These alloys of pure metals cannot meet the requirements of the surface of an electrode, which is used for obtaining caustic solution and hydrogen, i.e.
a. a non-porous coating on the cathode side must be guaranteed to prevent diffusion of gaseous or atomic hydrogen and caustic solution which usually can be found on the cathode in concentrations of 5 to 30 percent; PA0 b. the coating must be sufficiently "dense" to prevent diffusion of atomic hydrogen; PA0 c. corrosion protection must be optimum with regard to the following solutions: PA0 d. the coating must be electrochemically suitable for obtaining hydrogen from aqueous solutions, particularly from aqueous caustic solution; PA0 e. the coating must be very thin, approximately 0.2 - 1 mm.
Na OH -- 20 percent approx. PA1 Na Cl -- 0.1 - 2 percent approx. PA1 hypochlorite in small quantities
The electrodes that are already known meet the requirements of (d) and (c), as the coating on the cathode side is of pure metal or pure metal alloys. However, because of their metallic construction, they cannot adequately meet the requirements of (a), (b), and (c). Additionally, the electroplated galvanic coating has the disadvantage of a non-uniform electric field and uneven or even disengaged coating.